The delivery of therapeutic agents to mucosal surfaces has inherent difficulties due to the moist nature of the mucosal surfaces. This problem is particularly acute in the delivery of therapeutic agents to the surface of the eye where the washing effects of the tear film often removes much of the therapeutic agent. For glaucoma medications, for example, the result is a small pulse of drug is delivered to the target ocular tissues, while the majority of the drug is absorbed systemically.
The delivery of anionic therapeutic agents utilizing cationic polymers such as for example, cationic polysaccharides to bind the anionic therapeutic agent to the mucosal surface has been described, for example, in U.S. Pat. No. 5,358,706 issued Oct. 25, 1994. When the therapeutic agent is anionic, e.g., hyaluronic acid, the adhesion to the cationic substrate polymer is generally acceptable and the delivery of the anionic therapeutic agent to the mucosal surface can be controlled. However, in the case of cationic therapeutic agents, e.g., beta-blockers, the naturally occurring attractive forces between the positive and negatively charged species in the cationic/anionic systems is absent. Thus, both the adhesion to the cationic polymer substrate and the control of the delivery of the cationic therapeutic agent to mucosal surface can be uncertain.
Accordingly, there is a need for a method for treating mucosal surfaces such as, for example, the eye, in order to retain an active agent for a longer period of time. Moreover, there is a continuing need to produce ophthalmic formulations that are characterized as long lasting (sustained release) and are comfortable to the eye.